From: Mark Kirkwood <markir@paradise.net.nz>
To: Kevin Brown <blargity@gmail.com>
Cc: pgsql-performance@postgresql.org
Subject: Re: Simple Join
Date: Wed, 21 Dec 2005 19:55:20 +1300
Message-ID: <43A8FC58.1020609@paradise.net.nz> (raw)
In-Reply-To: <43A0AB0A.3000403@paradise.net.nz>
References: <200512141603.52583.blargity@gmail.com>
<43A0AB0A.3000403@paradise.net.nz>
Mark Kirkwood wrote:
> Kevin Brown wrote:
>
>> I'll just start by warning that I'm new-ish to postgresql.
>>
>> I'm running 8.1 installed from source on a Debian Sarge server. I
>> have a simple query that I believe I've placed the indexes correctly
>> for, and I still end up with a seq scan. It makes sense, kinda, but
>> it should be able to use the index to gather the right values. I do
>> have a production set of data inserted into the tables, so this is
>> running realistically:
>>
>> dli=# explain analyze SELECT ordered_products.product_id
>> dli-# FROM to_ship, ordered_products
>> dli-# WHERE to_ship.ordered_product_id = ordered_products.id AND
>> dli-# ordered_products.paid = TRUE AND
>> dli-# ordered_products.suspended_sub = FALSE;
>
>
> You scan 600000 rows from to_ship to get about 25000 - so some way to
> cut this down would help.
>
> Try out an explicit INNER JOIN which includes the filter info for paid
> and suspended_sub in the join condition (you may need indexes on each of
> id, paid and suspended_sub, so that the 8.1 optimizer can use a bitmap
> scan):
>
>
> SELECT ordered_products.product_id
> FROM to_ship INNER JOIN ordered_products
> ON (to_ship.ordered_product_id = ordered_products.id
> AND ordered_products.paid = TRUE AND
> ordered_products.suspended_sub = FALSE);
It has been a quiet day today, so I took another look at this. If the
selectivity of clauses :
paid = TRUE
suspended_sub = FALSE
is fairly high, then rewriting as a subquery might help:
SELECT o.product_id
FROM ordered_products o
WHERE o.paid = TRUE
AND o.suspended_sub = FALSE
AND EXISTS (
SELECT 1
FROM to_ship s
WHERE s.ordered_product_id = o.id
);
However it depends on you not needing anything from to_ship in the
SELECT list...
Cheers
Mark
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To: pgsql-performance@postgresql.org
Cc: markir@paradise.net.nz, blargity@gmail.com
Subject: Re: Simple Join
In-Reply-To: <43A8FC58.1020609@paradise.net.nz>
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